The invention concerns a method for the preparation of leather and fur skins. In the prior art, raw hides or raw skins are soaked in a medium containing water as the primary component and are conventionally prepared for pickling and subjected to other beamhouse (wet) operations such as cleaning and liming. Next, the hides or skins are partially freed of the water contained therein or adhering thereto by centrifuging, squeezing out, or draining. Then they are placed in a liquid medium in a pit or drum where they are subjected to the reaction with pickling chemicals and tanning chemicals and if necessary are treated with a lubricant. Then, in the case of leather preparation, the thus tanned leather is subjected to post-treatment or post-tanning in the liquid medium by the reaction of neutralization chemicals, if need be tanning chemicals, or fatliquor. In the practical application of this known process, aqueous media have been used exclusively up to now for the preparation of leather. The same is true also for the preparation of fur hides (see for example, Fritz Stather, Gerberei-Chemie und Gerberei-Technologie [Tanning Chemistry and Tanning Technology], 4th Ed., page 340, and Hubert Riskes, Die Pelzwirtschaft [The Fur Business], No. 4, 1974, page 42 ff).
Originally aqueous media were used which contained the chemicals and agents necessary for carrying out the individual processing steps in only small concentrations, preferentially in the form of weak solutions. A disadvantage of the known method is the extensive pollution of the environment caused by the discharge of the aqueous media which still contains chemicals used for treatment as well as by the waste water used for intermediate rinsing processes. There are special problems when waste waters containing chromium are present; such are employed in tanning or finishing high-temperature-resistant leather or fur skins. Since these waste waters are produced in large quantities, a high economically unjustified expenditure is required to convert them to a problem-free dischargeable form. There is also the disadvantage (in the known process) that a considerable amount of chemicals in the production process do not react, but are discharged with the waste waters.
Recently attempts have been made to reduce the quantities of waste waters to be discharged by increasing the concentration of the chemicals used in the aqueous media and by eliminating rinsing processes between the individual processing steps. New tanning chemicals as well as new equipment have been used for this purpose, for exmple, the Coretan machine, the Hagspiel machine, or devices of a cement-mixer type used for the tanning of leather, and cement-mixer type devices or drums, for example, for the preparation of fur skins. While the waste water problem and the loss of chemicals can be improved somewhat through such measures, there remains, however, a large clearly technical expenditure that must be made in order to be able to discharge the waste waters particularly conditioned by tanning in a composition which does not endanger the environment. In many countries maximum permissible chemical quantities which can be discharged have been established by law.
Fatliquoring, degreasing, and dyeing of leather and fur skins have already been conducted in organic solvents. It has also been proposed (see Stather, Gerberei-Chemie und Gerberie-Technologie, 4th Ed., page 319, and H. Weber, Das Leder [Leather], 1969 Annual, p. 8ff) that tanning be conducted with solutions of water-soluble (miscible with water) alcohols and ketones, for example, acetone. Since hides and skins are hydrophilic and contain water, experts have earlier been prejudiced to limiting such media to the use of organic solvents that are miscible with water. It was thought that only in this way could the solvents and the chemicals dissolved in them penetrate the material to be tanned. But such tanning process could not be carried out in practice because of the fire hazard of solvents which are water-miscible and also because of the high costs required for solvent recovery.
It is an important object of the present invention to prepare leather and fur skins in a way which permits a great reduction of harmful waste waters or waste waters difficult to convert to harmless form.
It is a further object of the invention to considerably economize on the cost of chemicals, thus leading to a shortening of processing time consistent with the preceding object.
The invention is to be incorporated in and conducted economically in conventional processes thereby saving equipment costs consistent with one or more of the preceding objects.
It is a further object of the invention to reduce energy costs in the drying process, and to create at least a partial savings in the milling process for skin preparation consistent with one or more of the preceding objects.